What Causes Pain Under Diaphragm Radiating To Back With History Of Whipple Surgery?

My question is i have pain on the left side under diaphram and pain radiates to the back. It is on and off. I have had it after I had whipple surgery. It has gotten worse. I had a CT scan recently, no tumor was found in that area. The doctor doesn't know what caused the pain. Can you explain to me why I have this pain and it is on and off.